Apparently a RAIDZ2 with 9 disks is computationally expensive: my system got as low as 34% CPU Idle when doing one disk rebuild (and one other disk offline). The CPU isn’t exactly fast (Athlon 64 X2 4600+) but I didn’t expect the usage to be this high. I’ll see how it goes after rebuild finished. Good thing I cancelled my plan of using an Atom for my other file server.

last pid: 62412;  load averages:  1.34,  1.28,  1.25                                                                   up 0+02:39:21  01:32:32
54 processes:  1 running, 52 sleeping, 1 zombie
CPU:  0.0% user,  0.0% nice, 63.2% system,  1.9% interrupt, 34.9% idle
Mem: 46M Active, 32M Inact, 4654M Wired, 104K Cache, 85M Buf, 1160M Free
Swap: 400M Total, 400M Free